Electric power conversion systems are used to condition the electric power supplied to motor load circuits from a direct current (DC) source of relatively constant voltage. If supplying DC motors, such a system will include an electric power "chopper" that is suitably controlled to vary the magnitude of load current and/or voltage as desired. Alternatively, in the case of alternating current (AC) motors, the system will include an electric power "inverter" that is suitably controlled to vary the amplitude and frequency of load voltage as desired. In either case, electric power flows from the DC source terminals to the load terminals of the controllable converter during "motoring" operation or in a reverse direction during "electrical braking".
Such a system is useful for propelling a rapid transit vehicle, in which case the source comprises a wayside conductor and the load comprises windings of at least one traction motor whose rotatable shaft is mechanically coupled through torque-increasing gearing to an axle-wheel set of the vehicle. The wayside conductor is typically energized by a relatively low voltage DC power generating plant located near the right of way along which the vehicle travels. In its motoring or propulsion mode of operation, the converter is so controlled that the DC voltage applied to its source terminals is converted into adjustable voltage at its load terminals, and the traction motor(s) responds by producing torque to accelerate the vehicle or maintain its speed as desired.
In the alternative electrical braking or retarding mode of operation of the power conversion system, the converter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the converter and appears as direct and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motor(s) responds by absorbing kinetic energy and slowing the vehicle. Electrical braking is achieved by a combination of dynamic braking and regenerative braking. Dynamic braking is effected by connecting a dynamic braking resistance between the DC source terminals. This resistance receives current from the converter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning to the DC power source power flowing in a reverse direction through the converter during braking operation. These two electrical braking modes can be combined in desired proportions, this mixing process being commonly referred to as "blending".
A power conversion system including a voltage source inverter for supplying AC traction motors is disclosed in U.S. Pat. No. 3,890,551--Plunkett, assigned to General Electric Company. An important feature of the Plunkett power conversion system is its inclusion of ohmic resistance (shown at 28 in FIG. 1 of the Plunkett patent) that is inserted into the DC link between the inverter and the DC power source during electrical braking but is effectively removed from the DC link during motoring. By inserting this series resistor during electrical braking, the magnitude of voltage at the DC terminals of the inverter can increase above that of the source voltage. One of the advantages of raising the inverter voltage is to enable the traction motors to develop more magnetic flux for braking and to use less current than would otherwise be required for very high braking effort.
The power conversion system of the Plunkett patent also includes a low pass electrical filter of the conventional series inductance (L), shunt capacitance (C) type between the voltage raising resistor and the inverter for attenuating harmonics generated by operation of the inverter and for partially isolating the inverter from undesirable line transients. (As used herein, the term "harmonics" refers to various components of the composite current and voltage waveforms having frequencies that are multiples of the frequency of the fundamental component of such waveforms.) In addition, the shunt capacitance of the filter at the DC terminals of the inverter provides the "stiff" voltage required for proper operation of a voltage source inverter.
The desired blending of dynamic and regenerative braking can be accomplished in various different ways that are well known to persons skilled in the art. See, for example, U.S. Pat. No. 4,093,900--Plunkett. In the present state-of-the-art, it is preferable to replace the parallel array of separate braking resistors and their respectively associated electromechanical switches, as shown in U.S. Pat. No. 4,093,900, with a single bank of resistance elements connected to the DC link via an electric power chopper comprising a controllable solid-state electric valve that can be repetitively turned on and off in a pulse width modulation (PWM) mode to control the average magnitude of current in the resistor as desired. An example of this modern practice is disclosed in U.S. Pat. No. 4,761,600--D'Atre et al., where the electric valve comprises a main thyristor for commutating the main SCR from a conducting state (on) to a non-conducting or current blocking state (off). Alternatively, a solid-state gate turn-off device (GTO) could be substituted for the chopper shown in U.S. Pat. No. 4,761,600.
One of the primary functions of the filter capacitors, in addition to "smoothing" the DC link voltage, is to reduce certain frequencies of current which can be introduced to the wayside conductors DC power source from the propulsion system. As is well known, such wayside conductors are often positioned adjacent wayside signalling equipment in transit applications. The signalling equipment may operate at preselected frequencies, such as, for example, 25 Hz, 60 Hz, 95 Hz, 200 Hz, or such other frequency as the transit authority may select. The signalling system may be used for communication to transit vehicles operating in the system or to indicate the presence of a transit vehicle within a particular block of the transit system. Other frequencies, such as 360 Hz, 720 Hz, and 990 Hz, are used for safety checks. Because of the importance of the signals on the signalling system, it is desirable that transit vehicles not generate signals in their respective propulsion systems which might interfere with the signalling system. To this end, the values of the capacitance means and the inductance means in the power filter circuit are selected to avoid oscillations or ringing at signalling frequencies or harmonics of these frequencies.
Notwithstanding the use of filter circuits to reduce electromagnetic interference (EMI) which might detrimentally affect the wayside communication system, it is also desirable to attempt to reduce the generation of such EMI at its source. As discussed above, one significant source of EMI is traceable to high currents in the dynamic braking grid resistance during electrical retarding of the vehicle. Modulation of these currents by turning choppers on and off produces harmonic currents in the resistance grids. The flux fields produced by these harmonic currents induce corresponding currents in the wayside conductors and rails which can interfere with wayside signaling systems. Accordingly, it is desirable to provide a method and apparatus to minimize induced currents in wayside conductors and rails.